Making Case.run more testable

[john-science]

Description

While looking at a possible duplicate coveragerc file, I noticed that the important Case.run() method had no code coverage because it was so tremendously long it was untestable.

So I split the really long method out into something with helper methods, so it could be at least partially testable.

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Checklist

• [X] This PR has only one purpose or idea.
• [X] Tests have been added/updated to verify that the new/changed code works.

• [X] The code style follows good practices.
• [X] The commit message(s) follow good practices.

• [X] The release notes (location doc/release/0.X.rst) are up-to-date with any bug fixes or new features.
• [X] The documentation is still up-to-date in the doc folder.
• [X] The dependencies are still up-to-date in setup.py.